Fix template clusterName behavior

[lcrisci]

This commit will allow you to define clusterName from the template's values file.
Before that our clusterName value would be overriden by the cli flag ( whether defined or not ).

Now we have an approach where if the value is defined, it will have priority over the cli flag.
This commit will also align with the official documentation under https://github.com/kubernetes/kops/blob/master/docs/cluster_template.md

[k8s-ci-robot]

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[lcrisci]

This pr would address the following issue: #5454

[mikesplain]

/ok-to-test

[johngmyers]

/retest

[johngmyers]

/lgtm

[rifelpet]

This looks good, thanks! I think technically we are changing behavior although previously we were just ignoring the value if it was set. Its probably worthy of a clear release note rather than being buried in the PR list. I can open that in a followup PR.

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lcrisci, rifelpet

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